1. Field of the Invention
The present invention relates to a communication system, and more particularly to an apparatus for amplifying multi-mode supporting power and a method for the same, which can operate as both a single carrier power amplifier and a multi-carrier power amplifier by improving linearity of an output signal in a communication system.
2. Description of the Related Art
In a communication system, power amplifiers are disposed at final output parts of Base Stations (BSs) and Mobile Stations (MSs), so as to amplify an input signal of a low level, for example, a base band signal, to an output signal of a high level, for example, a Radio Frequency (RF) band signal.
The RF signal having an amplitude modulated by a power amplifier usually has a distorted signal component due to non-linearity of the power amplifier. Such non-linearity of the power amplifier generates other frequency components in addition to the input frequency component that influence adjacent channels. That is to say, the gain reduction by the input signal and the channel interference due to inter-modulation degrade the capacity of the entire system. Therefore, linearity of the power amplifier is a very important parameter in the power amplifier.
Specifically, non-linear distortion of the power amplifier may cause interference between signals of reception bands or between channels. Therefore, there has been a requirement for a power amplifier having little distortion. There has been a method of increasing the quantity of current used by amplifier elements in order to reduce the distortion. However, according to this method, the quantity of used current may become too large in a high output power system. Therefore, various methods for reducing the distortion elements through application of distortion compensation circuits have been put to practical use.
For example, various schemes including a feedforward scheme, a feedback scheme, a pre-distortion scheme, etc. may be used in order to increase linearity of the power amplifier. According to the feedforward scheme, only non-linear components are detected from an output of a non-linear power amplifier and are then offset in an output signal. According to the feedback scheme, distortion or components causing the distortion are extracted at an output side and are then compensated for at an input side.
A power amplifier using the pre-distortion scheme in order to increase linearity of the power amplifier has a simpler structure than that of the feedforward scheme, because it does not require a separate amplifier for a distortion elimination loop. Therefore, a power amplifier using the pre-distortion scheme can be manufactured to have a small volume and has a high efficiency because it consumes less additional Direct Current (DC) power. Further, the power amplifier using the pre-distortion scheme can be manufactured at less manufacturing cost than the power amplifier using the feedforward scheme. In addition, the power amplifier using the pre-distortion scheme has an open loop structure and thus has less limitation in using the frequency bands than the power amplifier using the feedback scheme.
Therefore, typical communication systems usually use the pre-distortion scheme which allows for lower manufacturing costs and has less limitations in using the frequency bands due to the open loop structure than the feedback scheme.
The pre-distortion linearization scheme as described above refers to a scheme in which a pre-distortion circuit at a front end performs pre-distortion of a signal in order to achieve a characteristic opposite to the transfer function characteristic of a non-linear power amplifier, thereby linearizing the final output. There are various types of pre-distortion linearizers, which can be generally classified into analog linearizers and digital linearizers.
According to the digital linearization scheme, an amplitude and a phase of an input signal of a low level (for example, a baseband signal) are subjected to digital signal processing in a modem before modulation into an output signal of a higher level, for example, Intermediate Frequency (IF) or Radio Frequency (RF) modulation. Specifically, according to the digital linearization scheme, the amplitude and the phase of the input signal are pre-distorted by using an inverse function of a complex gain transfer function of a power amplifier. According to the digital linearization scheme, adaptive signal processing is easier than with other schemes because it uses a digital technology. However, according to the digital linearization scheme, a demodulation process for detection of an error signal is necessary, and it is difficult to compensate for change in a plurality of carriers at the final output part.
According to the analog linearization scheme, an input signal of a non-linear power amplifier is pre-distorted at an IF part or RF part, so as to implement the linearization. The analog linearization scheme is employed by various types of pre-distorters, such as a Bipolar Junction Transistor (BJT), a Laterally Diffused Metal Oxide Semiconductor (LDMOS) transistor, and a Gallium Arsenic (GaAs) Field Effect Transistor (GaAs FET), which can be selectively used to satisfy a specific purpose.
From among the various types of analog pre-distorters described above, an analog pre-distorter using the GaAs FET will be described hereinafter.
The analog pre-distorter using the GaAs FET generates an Inter-Modulation Distortion (IMD) component at a drive stage, which has the same amplitude as that of the IMD component at a main stage and an opposite phase to that of the IMD component at the main stage. By the generated IMD component, it is possible to eliminate the IMD component of the signal having passed through the main stage, to increase linearity of the power amplifier.
In a power amplifier including the pre-distorter described above, the LDMOS transistor employed in most power amplifiers is used at the main stage, while the GaAs FET, which can easily control change in the characteristic of the transistor by controlling the gate bias, is mainly used at the drive stage.
However, when the pre-distorter uses the GaAs FET, the IMD characteristic of the GaAs FET is usually set in accordance with the IMD characteristic of the main stage. Therefore, when input of the power amplifier changes, the operation area of the power amplifier changes, thereby changing the IMD characteristic of the main stage and degrading the performance of the pre-distorter. For example, when the input power is increased, the power amplifier operates in a non-linear area, thereby degrading the IMD characteristic. In contrast, when the input power is decreased, the IMD characteristic is improved.
Meanwhile, the power amplifier as described above can be classified according to the bandwidth of the input signal into a Single Carrier Power Amplifier (SCPA), which uses a single carrier, and a Multi-Carrier Power Amplifier (MCPA), which uses a multi-carrier. The SCPA amplifies the single carrier, that is, a 1FA signal. Therefore, the SCPA does not require linearization in order to secure linearity. The MCPA amplifies a multi-carrier, that is, 3FA signal or 8FA to 15FA signals Therefore, the MCPA requires linearization in order to eliminate inter-modulation components of a wideband.
Recent communication systems employ technologies for increasing transmission speed and capacity through beam forming, diversity, etc. by applying a multiple antenna system, for example, a smart antenna system. Because the multiple antenna technology uses a plurality of antennas, it requires the same output power for the entire power amplifier but requires a reduced output power for operation of each antenna in the power amplifier.
Meanwhile, due to its characteristics as described above, it is difficult to apply the analog pre-distorter using the GaAs FET to a power amplifier which can generate and output both a single carrier signal of a high output power and a multi-carrier signal of a low output power.
That is, if there were a single power amplifier which can generate and output both a single carrier signal of a high output power and a multi-carrier signal of a low output power, the single power amplifier can operate as an SCPA for amplifying only a single carrier at normal times and operate as an MCPA when it is applied to a multiple antenna system such as a smart antenna system. However, the conventional systems have failed to provide such a power amplifier due to the characteristics of the analog pre-distorter using the GaAs FET.